Apparatus for treating water and/or wastewater which utilize conventional membrane separation technology incorporating reverse osmosis and/or nano-filtration thin film membrane separation technology are well known and have been commercially available for many years. One example of an apparatus utilizing conventional membrane separation technology is shown in FIG. 1, defined hereafter, and outlined in general terms below.
Conventional membrane separation technology generally incorporates the following processes.
A raw (untreated) water source A is directed to the inlet of the conventional membrane separation device in either a flooded suction condition or under pressure by either a raw feed booster pump or via gravity feed.
A coarse filter B is placed prior to the conventional membrane separation device to separate large solids that might interfere with or damage subsequent pumps and/or membranes.
Pre-treatment apparatus C and devices are then placed to remove dissolved ions, polar contaminates and/or suspended contaminates that might cause damage to, or prevent the efficient operation of, the membrane separator. A partial list of common devices now used with conventional membrane separation technology, chemicals used, costs associated with the pre-treatment devices and the contaminates that they are designed to remove is shown below.
__________________________________________________________________________ DEVICE CHEMICALS USED COSTS CONTAMINATES __________________________________________________________________________ REMOVED MANGANESE GREENS POTASSIUM PERMANGANATE MEDIA REPLACEMENT, IRON, MANGANESE AND IRON FILTERS MECHANICAL UP-KEEP CHEMICAL FEEDS WATER WASTAGE ACIDIFICATION HYDROCHLORIC ACID, ACID FEED NONE, ALLOWS OPERATION SULPHURIC ACID, MECHANICAL UPKEEP ON HARD WATER PHOSPHORIC ACID CHLORINE SAND CHLORINE COMPOUNDS MECHANICAL UP-KEEP, IRON, MANGANESE, FILTRATION IRON FILTERS MEDIA REPLACEMENT, BACTERIA, SOME CHEMICAL FEEDS ORGANICS WATER WASTAGE DECHLORINATION ACTIVATED CARBON, MEDIA REPLACEMENT, CHLORINE, OZONE, EQUIPMENT SODIUM THIOSULPHITE FEED CHEMICAL FEEDS, SOME ORGANICS WATER WASTAGE MECHANICAL UP-KEEP OZONATION DESICCANTS, OZONE DESICCANT UP-KEEP, IRON, MANGANESE, MECHANICAL UP-KEEP SOME ORGANICS, COLOR OZONE DESTRUCT SYSTEMS MEDIA SYSTEM MEDIA REPLACEMENT, OZONE MECHANICAL UP-KEEP SAND FILTERS NONE MEDIA REPLACEMENT, SUSPENDED SOLIDS, MECHANICAL UP-KEEP PARTICULATES, TURBIDITY WATER SOFTENER SODIUM CHLORIDE SALT, SALT FEED, CALCIUM, MAGNESIUM ION EXCHANGER POTASSIUM CHLORIDE SALT MEDIA REPLACEMENT, HARDNESS MECHANICAL UP-KEEP ORGANIC SCAVAGING SODIUM CHLORIDE SALT, SALT FEED, SOME ORGANICS, SOME ION EXCHANGER CAUSTIC SODA CAUSTIC FEED, FORMS OF SILICA MEDIA REPLACEMENT, COLOR, SULPHATES MECHANICAL UP-KEEP SODA ASH/LIME SODA ASH, SODA ASH FEED, CALCIUM, MAGNESIUM EXCHANGE SOFTENERS SODIUM CARBONATE SODIUM CARBONATE FEED HARDNESS, SUSPENDED MECHANICAL UPKEEP SOLIDS, PARTICULATES, FILTER BED UPKEEP TURBIDITY MEDIA REPLACEMENT __________________________________________________________________________
Fine polishing filters D, generally of the disposable cartridge type, are provided with smaller systems and/or automated sand filter/coagulant systems are provided with larger systems and follow the pre-treatment package. The purpose of this is to provide both filtration to below 15 microns and a silt density index (SDI) of below 5. These are the maximum levels normally acceptable with conventional membrane separation technologies.
A raw feed water shut-off valve E, usually automatic and controlled by the membrane separator device, is normally incorporated in order to prevent raw feed water from entering the membrane separator device when not in operation.
A raw feed water system pressure boost pump F or boost pump set is arranged following the shut-off valve to increase the pressure of the raw feed water to an acceptable level above the osmotic pressure of the raw feed water solution. Operating pressures vary from device to device. In general, the higher the total dissolved solids level of the raw feed water solution to be treated, the higher the operating pressure of the device. Common operating pressures are shown below.
__________________________________________________________________________ TOTAL DISSOLVED TYPICAL OPERATING FEED SOLUTION SOLIDS RANGE PRESSURE RANGE __________________________________________________________________________ LOW SALINITY 0 to 1,000 mg/l 60 to 150 psi SLIGHTLY BRACKISH WATER 1,000 to 5,000 mg/l 150 to 250 psi MODERATELY BRACKISH 5,000 to 10,000 mg/l 250 to 550 psi HIGHLY BRACKISH WATER 10,000 to 25,000 mg/l 450 to 850 psi SEA WATER 25,000 to 38,000 mg/l 800 to 1,150 psi EXTREME SALINITY 35,000 to 50,000 mg/l 900 to 1,850 psi __________________________________________________________________________
As higher operating pressures improve both the product water output of membrane separators operated in the conventional manner and the product water quality, higher pressures than those indicated for the level of dissolved solids present in the raw feed water are sometimes used, but operating at higher pressures results in higher operating costs per volume of product water recovered.
The raw feed water system pressure boost pump or pump set must produce both the pressure required to operate the membrane separator and the required flow as well. Most conventional membrane separator specifications will only allow 10 to 15% recovery of the raw feed water stream if rated membrane service life, final water quality parameters and membrane warranty conditions are to be met. Some conventional membrane separator system designs do not follow these specifications, but this is bad practice.
Pump/motor combinations may include air-cooled motors with positive displacement pumps, single stage centrifugal pumps, or multi-stage centrifugal pumps, or water-cooled submersed motors with multi-staged centrifugal pumps. Average motor efficiencies for these pump designs are as follows.
______________________________________ Air-Cooled Motor, Positive Displacement Pump 55% Air-Cooled Motor, Centrifugal Pump 60% Water Cooled, Submersed Motor, Centrifugal Pump 75% ______________________________________
The majority of conventional membrane separator designs operate with air-cooled motors. These are the least efficient and heat generated by the motor is lost to the atmosphere.
In order to meet the membrane separator warranty specifications, the system pressure booster pump/pump set must be capable of producing no less than 8, but preferably 10, times the anticipated flow of final recovered product water. The excess water may be discharged, creating a very water wasteful situation, or be partially recycled. In either case, the raw feed water main drive pump(s) must be capable of pressurizing the same volume of water. This involves considerable horsepower as shown below.
__________________________________________________________________________ FULL RECIRCULATION NO RECIRCULATION CONDITION: SEA WATER, 30,000 TDS SEA WATER, 30,000 TDS __________________________________________________________________________ OPERATION PRESSURE: 850 PSI 850 PSI REQUIRED PRODUCT WATER FLOW: 1 USGPM 1 USGPM REQUIRED MEMBRANE TOTAL: 10 USGPM 10 USGPM FEED FLOW TOTAL MAIN DRIVE PUMP FLOW 10 USGPM 10 USGPM RECIRCULATION FLOW: 8 USGPM 0 USGPM WASTE FLOW: 1 USGPM 9 USGPM __________________________________________________________________________